1. Field of the Invention
The present invention relates generally to semiconductor manufacturing and, more particularly, to the characterization and control of lithographic process conditions used in microelectronics manufacturing.
2. Description of Related Art
During microelectronics manufacturing, a semiconductor wafer is processed through a series of tools that perform lithographic processing, usually followed by etch or implant processing, to form features and devices in the substrate of the wafer. Such processing has a broad range of industrial applications, including the manufacture of semiconductors, flat-panel displays, micromachines, and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist (hereinafter, also referred to interchangeably as resist) film on a substrate. Those segments of the absorbed aerial image, whose energy (so-called actinic energy) exceeds a threshold energy of chemical bonds in the photoactive component (PAC) of the photoresist material, create a latent image in the resist. In some resist systems the latent image is formed directly by the PAC; in others (so-called acid catalyzed photoresists), the photo-chemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent etch processing, the resulting resist film pattern is used to transfer the patterned openings in the resist to form an etched pattern in the underlying substrate. It is crucial to be able to monitor the fidelity of the patterns formed by both the photolithographic process and etch process, and then to control or adjust those processes to correct any deficiencies. Thus, the manufacturing process includes the use of a variety of metrology tools to measure and monitor the characteristics of the patterns formed on the wafer. The information gathered by these metrology tools may be used to adjust both lithographic and etch processing conditions to ensure that production specifications are met. Control of a lithographic imaging process requires the optimization of exposure “dose” and “focus” conditions in lithographic processing of product substrates or wafers.
Lithographic systems consist of imaging tools that expose patterns and processing tools that coat, bake and develop the substrates. The dose setting on the imaging tool determines the average energy in the aerial image. Optimum dose produces energy equal to the resist threshold at the desired locations on the pattern. The focus setting on the imaging tool determines the average spatial modulation in the aerial image. Optimum focus produces the maximum modulation in the image. The settings of many other imaging and processing tool parameters determine the “effective” dose and defocus (deviation from optimum focus) that form the latent image in the resist film. For advanced imaging tools, such as step-and-scan exposure systems, imaging parameters that determine the effective dose and defocus include the dose setting, slit uniformity, mask-to-wafer scan synchronization, source wavelength, focus setting, across-slit tilt, across-scan tilt, chuck flatness, etc. For advanced processing tools, processing parameters that determine the effective dose and defocus include the coat thickness and uniformity, the post-expose bake time, temperature and uniformity, the develop time, rate and uniformity, wafer flatness, topography, etc. Typically, the different imaging and process parameter sources of variation can be distinguished by the spatial signature of the effective dose and defocus variation they cause.
Variation in both imaging and process parameters cause variations in the spatial distributions of effective dose and defocus in the resist film that, in turn, cause variations in the dimensions of the printed patterns. Because of these variations, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range. The importance of such monitoring increases considerably as the resolution limit, which is usually defined as minimum features size resolvable, of the lithographic process is approached. The process control objective is to detect and correct imaging and process parameter deviations from the nominal settings that have been determined to produce the desired pattern dimensions. Effective dose and defocus represent a consolidation, into two variables, of the large number of possible imaging and process parameter settings and the large number of possible pattern dimensions. Thus, the measurement and control of the distributions of effective dose and defocus is the most efficient and effective path to the optimization of the patterning process.
As described in U.S. Pat. Nos. 5,953,128; 5,965,309; 5,976,740; 6,004,706; 6,027,842 and 6,128,089, the effective dose and defocus of a lithographic image can be measured using dual-tone optical critical dimension (OCD) metrology; however, as disclosed in U.S. application Ser. No. 09/765,148 by the instant inventor, the dual-tone approach necessitates the use of intentional focus offsets (a.k.a., “canaries”) to determine the sign of defocus. Canaries result in undesirable loss of pattern fidelity in focus-offset regions of the wafer. Thus, the problem remains of determining the sign and magnitude of both dose and defocus at each individual measurement site without affecting the patterning fidelity. These requirements would be desirable for an automated dose and focus control method and system.
While published PCT patent application no. WO 03/001297 A2 discloses that the effective dose and defocus of a lithographic image can be extracted from shape parameters such as linewidth, resist height, sidewall angle, top profile, bottom profile or resist loss, the disclosed method of doing so is not generally workable.